Superelastic 3D Assembled Clay/Graphene Aerogels for Continuous Solar Desalination and Oil/Organic Solvent Absorption

Abstract Superelastic, arbitrary‐shaped, and 3D assembled clay/graphene aerogels (CGAs) are fabricated using commercial foam as sacrificial skeleton. The CGAs possess superelasticity under compressive strain of 95% and compressive stress of 0.09–0.23 MPa. The use of clay as skeletal support significantly reduces the use of graphene by 50%. The hydrophobic CGAs show high solvent absorption capacity of 186–519 times its own weight. Moreover, both the compression and combustion methods can be adopted for reusing the CGAs. In particular, it is demonstrated a design of 3D assembled hydrophilic CGA equipped with salt collection system for continuous solar desalination. Due to energy recovery and brine transport management promoted by this design, the 3D assembled CGA system exhibits an extremely high evaporation rate of 4.11 kg m−2 h−1 and excellent salt‐resistant property without salt precipitation even in 20 wt% brine for continuous 36 h illumination (1 kW m−2), which is the best reported result from the solar desalination devices. More importantly, salts can be collected conveniently by squeezing and drying the solution out of the salt collection system. The work provides new insights into the design of 3D assembled CGAs and advances their applications in continuous solar desalination and efficient oil/organic solvent adsorption.


SEM Image of MF/RGO/Clay Composite
. SEM image of clay/RGO/MF composite. RGO sheets self-assembled into network structures in the skeleton of MF, preventing serious stacking of RGO sheets. 9 Figure S5. SEM images of GA prepared without using MF as sacrificial skeleton. Figure S6. Photos of clay/GO suspension and clay/RGO hydrogel without MF as a sacrificial skeleton. During the reduction process, the enhancement of π-π stacking interactions between RGO sheets, resulting in the volume shrinkage of clay/RGO hydrogel. 11 Figure S7. The densities of CGAs as a function of GO concentration. Figure S8. The schematic diagram to describe how the gap are repaired during a 3D CGA assembly process. MFs were immersed into the clay/GO mixed solution, followed by stacking together tightly. During the reduction-1 process, GO sheets reduced to RGO sheets, which selfassembled into RGO network throughout the gaps between adjacent MFs. During the reduction-2 process, the obtained clay/RGO/MF hydrogel was immersed in hydroiodic acid solution (HI) to remove the MF skeleton and further reduce the functional groups on RGO sheets, resulting in the formation of clay/RGO network in the gaps between adjacent MFs. The clay/graphene hydrogel was freeze-dried and thermally treated to obtain an assembled CGA with continuous network. 13 Figure S9. SEM image of gap junction cross-section of a 3D assembled CGA. After assembling into a 3D CGA, the gaps between MFs were completely repaired and presents a unique porous structure.   Figure S11. SEM image of AGA with low magnification. AGA shows a large-area (＞1 mm 2 ) uniform porous structure. Figure S12. TEM images of GO (a) and GA (b). The smooth surface of the GO sheets formed wrinkle regions after reduction process.   Figure S16. Compressive stresses of CGAs with different densities at 95% strain. Figure S17. The ultimate stresses of CGAs were compared with those of other elastic aerogels as a function of the maximum strain and density, respectively.  Figure S18. SEM image of AGA after a series of harsh tests.

SEM Image of AGA After a Series of Harsh Tests
27 Figure S19. Fast separation of toluene (dyed with Sudan III) from water with the 3D cubic cupshaped MGA with within several seconds. When the toluene (dyed with Sudan III)/water mixture was poured into the cubic cup-shaped MGA, the toluene was rapidly absorbed, resulting in a complete separation of water and toluene within several seconds.

Fast Separation of Organic Solvent/Water with 3D Assembled MGA
28 Figure S20. Adsorption capacities of the GA measured for a range of organic solvents in terms of their densities.

Schematic Illustration for The Experimental Setup of The Solar Steam Generation
Test Figure S21. Schematic illustration for the experimental setup of the solar steam generation test of AGA.
32 Figure S22. Based on the skeleton of 3D MF, a 3D AGA Ⅰ (30 mm × 30 mm × 20 mm) was fabricated. Figure S23.  Figure S26. Schematic illustration of the continuous desalination test for the 3D AGA Ⅲ under 1 and 3 sun illumination. Due to the cubic-cup part of the 3D AGA Ⅲ was the same in crosssection perpendicular to brine flow, the brine flowed along the cubic-cup walls. Under one sun illumination, salt accumulation occurred on the upper surface of the 3D AGA Ⅲ

Schematic Illustration of the Continuous Desalination Test for the 3D AGA Ⅲ Under 1 and 3 Sun Illumination
. With the enhancement of solar-driven evaporation, salt gradually accumulated on the cup walls of the 3D AGA Ⅲ under 3 sun illumination.

The Evaporation Performance of 3D AGA Ⅳ in 20 wt% Brine Under Varied
Illumination Intensity Figure S29. The mass change of water versus time with 3D AGA Ⅳ in 20 wt% brine under varied illumination intensity. Figure S30. The p-MF has a hole with the same size as the inner diameter of the 3D AGA Ⅳ to allow sunlight to pass through. Figure S31. Schematic of preparation process of p-MF. Pristine MF (H1: 20 cm) was pressed under 230 o C for 15min to obtain p-MF (H2: 2 cm). Figure S32. Photographs of (a) pristine MF and (b) p-MF. SEM images of (c) pristine MF and (d) p-MF.

Photos and SEM Images of Pristine MF and p-MF
43 Figure S33. The photograph of pristine MF and p-MF after red ink adsorption. P-MF showed better fluidic transport property than pristine MF. surface of the 3D AGA IV is lower than the ambient temperature. Therefore, the 3D AGA IV can harvest energy from the environment to enhance evaporation performance.

Photographs of the Absorption of Brine by p-MF
46  Figure S36. Mass change of brine (20 wt%) for continuous 36 h illumination (one sun). 3D

Continuous Desalination Test
AGA Ⅳ equipped with p-MF showed a high average evaporation rate of ~4.10 kg m -2 h -1 .
49 Figure S37. Photographs of the p-MF-equipped 3D AGA Ⅳ at 12, 24, and 36 h during the continuous desalination test under 1-sun illumination, respectively. No salt accumulation was observed on the inner and outer surface of the 3D AGA Ⅳ during continuous desalination process.

Continuous Desalination Test
50 Figure S38. Photographs of precipitated salt collected by drying the brine solution in p-MF every 9 h. 55  GA (with MF as sacrificial skeleton) and clay/RGO aerogel (without MF as sacrificial skeleton) was also prepared by the aforementioned method.

Fabrication of 3D CGAs
A piece of MF was cut into a 3D shape and 3D CGA was prepared by the aforementioned method (2.2 Fabrication of CGA).
As shown in Figure 5e, 3D AGA Ⅰ (30 mm × 30 mm × 20 mm) was fabricated by using a cubic cup shaped MF.

Fabircation of 3D Assembled CGAs
Several MFs were immersed into the above mixed solution by a squeezing procedure for several times. These clay/RGO/MF composites were then kept close together during the preparation process. Finally, 3D assembled CGA with different shapes can be flexibly fabricated.
As shown in Figure 5h-i, 3D AGA Ⅲ (30 mm × 30 mm × 75 mm) was fabricated by using a pyramidal MF and a cubic cup shaped MF.

Construction of 3D Assembled CGA Equipped with p-MF
As shown in Figure S30,

Atomic Force Microscope (AFM)
Atomic force microscopy (AFM) images were recorded under ambient conditions using a Digital Instrument Multimode Nanoscope IIIA operating at a tapping mode. Samples were prepared by spin-coating a dispersion of GO (~ 0.3 mg mL -1 ) onto a freshly cleaved mica surface, respectively. The average size of the GO sheets is ~ 5 µm and the height difference between the steps is ∼ 0.9 nm, respectively ( Figure S2).

Scanning Electron Microscopy (SEM)
SEM images were obtained on a field-emission scanning electron microscope (Supra 55, ZEISS, Germany) using an accelerating voltage of 10 kV (Figure 2a-c, Figure S3-S5, S9-S11, S13, S18, S32c-d, and S41). All samples were spray-coated with a thin gold layer in vacuum prior to the SEM observations.

Measurements of Nitrogen Adsorption
Nitrogen adsorption measurements were performed with a Micromeritics TriStar II 3020 ASAP (Micromeritics, USA) to obtain pore properties such as the BET-specific surface area, 60 pore size distribution, and total pore volume ( Table S1). Before measurement, the samples were outgassed under vacuum at 100 °C for 10 h until the pressure less than 0.665 Pa.

Fourier Transform Infrared (FTIR) Spectroscopy
Fourier transform infrared spectra (FTIR) were measured in the range of 600 -4000 cm -1 with a NICOLET 6700 produced by Thermal Fisher Scientific Corporation (Figure 2j-l).

Thermal Gravimetric Analysis (TGA)
Thermal gravimetric analysis (TGA) was carried out using a thermogravimetric analyzer (TGA8000, PerkinElmer, USA) from 50 to 700 °C at a heating rate of 10 °C min -1 under a N2 atmosphere ( Figure S15). Before the measurements, all samples were dried in vacuum at 50 °C for 12 h.

Compression Testing
The compressive tests were performed with a rheometer (MCR 302, Anton Paar, Austria) using a 50 N load cell in the axial-compression testing mode at a strain rate of 10 mm min -1 .

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The sessile drop method was applied to measure the water contact angle using a contact angle instrument (OCA20, Dataphysics, Germany) (Figure 4a).

Measurement of the Solvent Adsorption Capacity
To measure the adsorption capacity of the samples for various solvents, samples were placed inside the solvents for a period of time (~30 s) and then were taken out for measurements. The weights of samples before and after adsorption were measured for calculating the adsorption capacity (Figure 4b-c, Figure 4f-h, Figure S20, and Table S4-S5).

Optical Measurement
The optical transmittance (T) and reflectance (R) spectra were measured in the range of 500 -2500 nm with a spectrophotometer (UV3600, Shimadzu, Japan) attached to an integrating sphere (ISR-3100) (Figure 5a).

Thermal Conductivity Measurement
The thermal conductivity measurement was performed on a thermal conductivity tester (HFM436, Netzsch, Germany) (Figure 5b).

Evaluation of Solar-driven Steam Generation in Laboratory
Solar simulator (CEL-S500-T5) with an optical filter for the standard AM 1.5 G spectrum was used to simulate sunlight. The water evaporation rate was evaluated by measuring the weight loss of water with an electrical balance (FA 2004, 0.1 mg in accuracy). We use the top projected plane of sample for the light intensity measurement and the radiation intensity is corrected by an optical power meter. An IR camera (FLIR E5xt) was utilized to measure and record the temperature changes. The mass change was measured by an electrical balance and then communicated to a laptop computer in real-time for the evaluation of the evaporation rate 62 and solar-thermal conversion effciency. The environmental temperature was ~25 °C and the relative humidity was ~ 40%. Note the evaporation rates of all 3D AGA Ⅳ equipped with p-MF were calculated by subtracting the evaporation rate of p-MF under one sun illumination (0.38 kg m -2 h -1 ).

Measurement of Ion Concentration
The ion concentrations of gathered fresh water were measured by the inductively coupled plasma emission spectrometer (ICPE-9820, Shimadzu, Japan) (Figure 6i-j and Table S7-8).